WIP fix runBackground with -i/--no-server (#4259)

This might have been the cause of a lot of flakiness that seems to have
gone away with #4254, as the
server exiting caused the `runBackground` calls to exit causing the http
servers to exit and fail to pick up requests.

Might have been caused by com-lihaoyi/os-lib#324
which made `destroyOnExit` the default for spawned subprocesses. This PR
explicitly disables `destroyOnExit` for the subprocesses where
`background = true`

Covered by a new `integration.invalidation` test that runs under both
`server` and `fork`, that previously failed when run under `fork`

integration/invalidation/run-background/resources/build.mill

@@ -0,0 +1,6 @@
+package build
+import mill._, javalib._
+
+object foo extends JavaModule{
+  def runBackgroundLogToConsole: Boolean = false
+}

integration/invalidation/run-background/resources/foo/src/foo/Foo.java

@@ -0,0 +1,20 @@
+package foo;
+import java.io.RandomAccessFile;
+import java.nio.channels.FileChannel;
+import java.nio.file.*;
+public class Foo {
+    public static void main(String[] args) throws Exception{
+        System.out.println("Hello World A!");
+        RandomAccessFile raf = new RandomAccessFile(args[0], "rw");
+        System.out.println("Hello World B!");
+        FileChannel chan = raf.getChannel();
+        System.out.println("Hello World C!");
+        chan.lock();
+        System.out.println("Hello World D!");
+        while(true){
+            if (!Files.exists(Paths.get(args[1]))) Thread.sleep(1);
+            else break;
+        }
+        System.out.println("Hello World E!");
+    }
+}

integration/invalidation/run-background/src/RunBackgroundTests.scala

@@ -0,0 +1,58 @@
+package mill.integration
+
+import mill.testkit.UtestIntegrationTestSuite
+import java.io.RandomAccessFile
+import utest.asserts.{RetryInterval, RetryMax}
+import scala.concurrent.duration._
+import utest._
+
+// Make sure that `runBackground` subprocesses properly outlive the execution
+// that created them, and outlive the Mill process whether terminated naturally
+// at the end of `--no-server` or terminated explicitly via `shutdown`
+object RunBackgroundTests extends UtestIntegrationTestSuite {
+  implicit val retryMax: RetryMax = RetryMax(5000.millis)
+  implicit val retryInterval: RetryInterval = RetryInterval(50.millis)
+
+  def probeLockAvailable(lock: os.Path): Boolean = {
+    val raf = new RandomAccessFile(lock.toIO, "rw");
+    val chan = raf.getChannel();
+    chan.tryLock() match {
+      case null => false
+      case locked =>
+        locked.release()
+        true
+    }
+  }
+
+  val tests: Tests = Tests {
+    test("simple") - integrationTest { tester =>
+      import tester._
+      val lock = os.temp()
+      val stop = os.temp()
+      os.remove(stop)
+      eval(("foo.runBackground", lock, stop))
+      eventually { !probeLockAvailable(lock) }
+      if (tester.clientServerMode) eval("shutdown")
+      continually { !probeLockAvailable(lock) }
+      os.write(stop, "")
+      eventually { probeLockAvailable(lock) }
+    }
+    test("clean") - integrationTest { tester =>
+      if (!mill.main.client.Util.isWindows) {
+        import tester._
+        val lock = os.temp()
+        val stop = os.temp()
+        os.remove(stop)
+        eval(("foo.runBackground", lock, stop))
+        eventually {
+          !probeLockAvailable(lock)
+        }
+
+        eval(("clean", "foo.runBackground"))
+        eventually {
+          probeLockAvailable(lock)
+        }
+      }
+    }
+  }
+}

main/util/src/mill/util/Jvm.scala

@@ -395,7 +395,8 @@ object Jvm extends CoursierSupport {
       env = envArgs,
       stdin = if (backgroundOutputs.isEmpty) os.Inherit else "",
       stdout = backgroundOutputs.map(_._1).getOrElse(os.Inherit),
-      stderr = backgroundOutputs.map(_._2).getOrElse(os.Inherit)
+      stderr = backgroundOutputs.map(_._2).getOrElse(os.Inherit),
+      destroyOnExit = backgroundOutputs.isEmpty
     )
   }